Porous polymeric structures having pores in the range of 0.01 to 10 microns are commonly used for microfiltration. Such membrane structures may be prepared from thermoplastic polymers using precipitation techniques and formed into various shapes including hollow fibres or flat sheets.
The thermal precipitation technique for membrane formation commences with the formation of a solution of a thermoplastic polymer in a solvent at an elevated temperature. The solution is then cooled and, at a specific temperature which depends on the polymer, the solvent, the concentration of the polymer in the solvent and the rate of cooling, phase separation occurs and the liquid polymer separates from the solvent.
True solutions require that there be a solvent and a solute. The solvent constitutes a continuous phase, and the solute is distributed randomly in the solvent at a molecular level. Such a situation is almost unknown with polymer solutions. Long polymer chains tend to bend back on themselves and form temporary interactions or bonds with other polymer chains with which they come into contact. These interactions are continually forming and breaking, and new ones are formed. Polymer solutions are thus rarely true solutions but lie somewhere between true solutions and mixtures.
In many cases it is also difficult to state which is the solvent and which is the solute. In the art, it is accepted practice to call a mixture of polymer and a solvent a solution if it is optically clear without obvious inclusions of either phase in the other. Phase separation is usually then taken to be that point where there is an optically detectable separation.
There is yet another case where the heated mixture of polymer, solvent, and other components if present, if neither a solution nor a mixture in the usual sense of the words. This is the case where a surface-active agent is present in sufficient concentration to form ordered structures such as micelles.
A hollow fibre forming installation disclosed in U.S. Pat. No. 4,708,799 includes a vessel containing a heated solution of a polymer and a liquid which is inert relative to the polymer which is delivered to an extrusion die by a pump. The hollow fibre is extruded through a nozzle into a spinning tube containing inert liquid of the solution. The fibre and the inert liquid pass through an atmospheric gap to a spinning tube in the same direction and at substantially the same linear velocity.
In that installation, the hollow fibre is extruded in the molten state and formed in the spinning tube. The application of the inert liquid to the molten fibre has an adverse effect on the porosity of the surface of the fibre. Furthermore, the use of a spinning tube is cumbersome and costly.